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Making Concrete with Carbon Dioxide

Injecting CO2 directly into ready mix trucks allows cement reductions

Concrete producers are increasingly being asked to provide mixes with a low-carbon footprint, especially for LEED-certified projects. Using fly ash as a substitute for portland cement is one approach, but with the current shortages of fly ash that has become increasingly difficult.

Another approach that has gained a lot of interest across the United States and Canada is the CarbonCure Ready Mixed Technology. Not only does this process consume CO2, but the compressive strength improvements from an optimized injection of CO2 enable the production of concrete with a reduced binder content without sacrificing performance.

This case study analyzes data collected by a producer using the CarbonCure Ready Mixed Technology to inject CO2 during normal concrete operations. This process uses controlled doses of carbon dioxide injected directly into the ready-mixed concrete truck during the initial batching and mixing. When liquefied CO2 is injected into wet concrete it chemically reacts with calcium ions released from cement to form solid, nano-sized calcium carbonate particles that become permanently bound within the concrete. The carbon dioxide binds as solid and stable carbonate reaction products in the cement matrix and provides a positive impact on the concrete properties.

In a process that resembles the introduction of a chemical admixture, the producer connected a tank of liquid CO2 to the CarbonCure injection system. This metered an optimum dose of CO2 into the drum of the ready mixed truck at the same time as the concrete was loaded. Upon entering the mixing drum, the liquid carbon dioxide converted into a mixture of CO2 gas and solid carbon dioxide snow whereupon it reacted with the hydrating cement to form solid calcium carbonate particles.

The concrete was then subjected to assessment and testing.

The producer conducted a three-way comparison among a standard mix, a mix with reduced-binder content, and a reduced-binder mix that used a dose of CO2. The binder reduction leads to a decrease in paste volume, but can also serve to slightly increase the water-to-cementitious-materials ratio and admixture loading per unit of binder. The former was expected to have a negative impact on strength development while the latter was expected to have a neutral impact.

Strength Enhancement Results

The average compressive strength measured for each batch at three test ages is summarized in Figure 1. The binder reduction in the non-air-entrained batch led to a strength reduction at all ages including a 17% drop in 28-day compressive strength. However, by adding the carbon dioxide, the strength of the reduced-binder batch improved to be within 4% of the standard mix at 28 days. This suggests that a CO2 injection in conjunction with a binder reduction on the order of 7% can result in concrete without compromised performance.

Figure 1: Compressive strength development of non air entrained (A) and air entrained (B) concrete test loads. The binder reductions were 8% and 7% for the non air entrained variations, respectively.

The binder reduction in the air-entrained batch led to an 11 to 13% drop in compressive strength across the three test ages. However, when the carbon dioxide was added the strength of the reduced-binder batch improved to be equivalent to the standard mix.

These results lead to two conclusions:

A reduction in the quantity of binder leads to a reduction in the compressive strength.

That strength reduction can be offset, however, by the introduction of CO2 into the concrete mix during batching.

Extended Production Results

The producer moved on to test these conclusions on actual production concrete mixes with 28-day design strengths of 3000, 5500, and 8000 psi using binder reductions of 4.5%, 4.4%, and 3.1%, respectively. In each case, the addition of an optimized dosage of CO2 was shown to bring the performance of the reduced-binder concrete mixes within the expected performance range of a mix with no binder reduction.

The success of these two assessments encouraged the ready-mixed concrete producer to apply the CarbonCure Ready Mix Concrete Technology across their concrete production. Over a 10-month period, spanning March to December, the producer injected CO2 into roughly 56,000 yd3 of concrete with an average cement reduction of 5%. This resulted in savings of 600 tons of cement and 530 tons of CO2 emissions with no reduction in the performance of the concrete.

This data shows that producers in full-scale operation can use the strength-enhancing effect of CO2 to reduce the amount of the most expensive ingredient in concrete while also reducing the carbon footprint of the mix.

CarbonCure’s retrofit technology chemically sequesters waste carbon dioxide during the concrete manufacturing process to make greener and stronger concrete. CarbonCure is part of a growing industry of CO2-utilization technologies that are expected to reduce global greenhouse gas emissions by 15% by 2030. CarbonCure’s technology is currently operational in a growing number of concrete plants across North America, including several of the world’s largest vertically-integrated cement and concrete companies. CarbonCure is one of 27 semi-finalists in the $20 million NRG COSIA Carbon XPRIZE challenge, which has been called the Nobel prize for climate technologies. For more information, visit: www.carboncure.com.